The broadest known process in plastic industry, employed in the manufacture of articles is the “injection” molding. Said process was first known in 40's decade and in present times a great number of variations are known, related to the type of products to be manufactured and the materials employed in their manufacture. Therefore, a number of specialized machinery has been designed to inject every type of plastic (thermoplastic and thermoset), rigid and flexible, usable in several applications and with several geometric shapes and also with different wall thicknesses.
In all cases, the “injection” molding processes, as well as other plastic molding processes are aimed to satisfy specific needs, based on the application of the molded product, with lowest possible costs, by developing an optimal application to comply with the specifications while reducing costs and manufacture or molding times.
In this connection, the wall thickness in a plastic product to be manufactured is one of the most important items to take into account in a molding process, since a lower wall thickness evidently implies a lower weight and therefore lower costs. Also a lower wall thickness requires lesser manufacture times as a shorter cycle will be required which means lower production costs. The opposite is also true: A greater wall thickness implies greater weight and greater costs as well as also greater manufacture times as a longer cycle will be required.
Wall thicknesses traditionally used in an “injection” molding process range between 0.5 mm and 3.0 mm. There are also special applications requiring of “thin walls” of less than 0.5 mm, for the manufacture of which specially designed machinery is needed. On the other side, there are also applications with walls of more than 3.0 mm; but in this case grave problems can be experienced as sink marks and deformations may be formed in the molded part; but normally can be reached wall thicknesses of up to 4.0/5.0 mm.
A so-called “structural foam injection” process has been developed for those molded parts having wall thicknesses greater than 5.0 mm, wherein a plastic material is injected inside the mold together with a foaming or expanding agent which, upon reacting, makes possible to maintain an inner mold pressure that prevents the formation of sink marks and deformations in the product. With this process, parts with a thickness greater than 10 mm or more can be injected without sink marks and deformations. However, the molding times are considerably increased since the cooling time within the mold must be enough to cool down said thickness.
Providers of machinery for the “structural foam injection” process have developed machines having a number of clamping units so that, while a mold is being cooled down, another part may be injected, thus increasing the productiveness of the process. However, a drawback is inherent in said machines: they are highly costly and a number of molds are required in order to carry out this process.
Accordingly, in present times, after great developments in the manufacture of machinery and equipment to inject plastics, two well known processes of public dominion can be found:
1. High-pressure injection, the traditional injection process, wherein parts with wall thickness of up to 4.0/5.0 mm are molded.
2. Low-pressure injection, also known as structural foam injection, wherein parts with wall thickness of more than 6.0 mm are molded.
In order to clear the applicability and the difference between both processes, an example will be given about the manufacture of a school desk paddle-board.